Turbomachines are in common use in a Diesel-engine environment. Diesel engines are found to require ever increasing turbocharger pressure ratios. Indeed, it has been estimated that every 10 years or so an increase of 0.75 bar is called for, largely as a result of increasingly stringent emissions regulations. As pressure ratio requirements increase, so stresses and temperatures on the impellers of turbochargers increase, which can affect the maximum operational lifespan of the impeller. Consequently there is a need to try to avoid keeping an impeller so long in service that it fails.
To achieve this, one approach has been to replace an impeller after a length of time which is assumed to represent the normal lifespan of the impeller, typically 50,000 hours. However, this can result in possible impeller failure if the impeller's life falls short of this figure. An alternative approach is to change the impeller components at regular conservative intervals. This, however, usually ignores the fact that, for part of their life within the turbocharger, these components are not in operation or are operating below their nominal duty profile (i.e. nominal number of hours at specific loads, numbers of cycles, etc.). Hence an additional cost is imposed on the turbocharger owner and manufacturer. Similarly, a manufacturer of a refurbished turbocharger will very often decide to install a new impeller in order to safely meet a minimum life requirement. This also will have a cost penalty where the impeller, which has been removed, is not yet at the end of its useful life.
A further measure sometimes taken to avoid failure of an impeller is to move away from the aluminium-based materials which are currently standard, to titanium-based materials offering improved mechanical strength. However, while this can provide a substantial margin of operational safety, it typically increases the cost of a turbocharger by 30%, which is clearly undesirable.
In some situations the problem of limited impeller life is simply ignored altogether, so that impellers are often run very close to their operational limit. This can run the risk of endangering life and property if the impeller fails, with the manufacturer's reputation suffering as a result.
A different approach, which avoids the drawbacks of the approaches already outlined, is to attempt to determine to as high a degree of accuracy as possible what fraction of the maximum lifespan of an impeller has expired at any particular point in time, so that an estimate can be made of the remaining lifespan. This knowledge enables as much of the impeller's total life as possible to be exploited.
The maximum lifespan of an impeller depends primarily on two limiting factors: creep and low-cycle fatigue. Creep is strongly influenced by the time spent at particular stresses and temperatures of the impeller, while low-cycle fatigue is influenced by the peak stresses and the cyclic duty of the impeller. The sites for creep and low-cycle fatigue do not normally coincide, though it is possible for creep to induce fatigue cracks. The stresses are primarily a function of the rotational speed of the impeller. However, the temperature distribution in the impeller can also produce stresses, which will add to the overall stress level. This is particularly exacerbated when the impeller operates in transient conditions, such as cold start and sudden load changes before the temperature distribution has reached steady state. Impeller temperature is a function of ambient temperature and impeller speed, while the cyclic duty of the impeller is a function of the number and nature of the variations in turbocharger speed. The damage which will be suffered by an impeller can be calculated from a knowledge of its duty cycle. This calculation is routinely carried out in the design of impellers, based on an estimated and perhaps an extreme duty, and a component life is then quoted. Typically this will be 50,000 hours, as mentioned earlier. In a similar manner, an estimate can be made of the amount of damage accumulated in a used impeller over a given period of time and a figure for elapsed lifespan derived. The remaining lifespan of the impeller can then be estimated on this basis.
An example of a safety design concept for the safe running of turbocharger impellers is the so-called “SIKO” program developed by ABB. This program, which is outlined in the publication “Turbocharger Maintenance: Optimizing Preventive Maintenance”, published in 2003 by ABB, is divided into a number of modules, namely:
determination of load profiles (turbocharger operating conditions);
determination of impeller material properties;
determination of stress and temperature distributions using 2D or 3D finite element analysis;
determination of cumulative damage using the linear Palmgren-Miner Rule; and
calculation of expired impeller lifespan from the cumulative damage.
Thus, on the basis of data regarding the impeller material properties, the stress calculations and the temperature distributions in the material, an impeller lifespan calculation can be carried out for a given load profile. The calculations are performed for each critical position of the compressor and turbine.
The use of sampled values of operational speed and temperature of a turbojet, or other type of engine, in order to derive the lifespan of a rotary component is also known from Russian patent SU 773657-B, published on 25 Oct. 1980.
The duty cycle of an impeller can be interpreted from the engine operating records, if any are kept. These records might include the speed of the turbocharger and the intake temperature and are kept within the engine management system and are therefore linked to the engine. Typically, records are kept for one operating point per day. In practice, turbochargers are routinely changed on an engine, either at regular intervals for maintenance purposes (e.g. every 15,000 hours) or as a result of an operational incident. The turbocharger which has been removed, and may still contain the same impeller, can then be used on a different engine, and possibly even on a different application (e.g. a power station, a marine engine, a locomotive, etc.), which may involve a different duty cycle.
Because of the high probability that an impeller or an entire turbocharger may be used on a number of different engines over its lifetime, it has proved very difficult to monitor its duty with any reliability over that period in order to make an accurate assessment of its remaining lifespan.
There is therefore a need to provide a way of assessing with greater reliability the elapsed lifespan of an impeller. In addition, it is desirable to be able to more accurately define the running conditions and cycles experienced by the turbocharger. These cycles can typically vary over a period of seconds rather than once per day, as mentioned earlier.